Servo valves are a class of devices used in hydraulic systems using electromagnetic control. Typically the electromagnetic stage is separate from a pilot stage that is used to generate fluid streams with differential pressures operable to move larger valves by applying the pressure differences to larger surfaces areas. An electromagnetic stage is used so electronic feedback control may be applied to the overall hydraulic system employing a servo valve. The electromagnetic stage is typically a solenoid, torque motor, or force motor structure. In solenoids, the force allows a cylindrical element to be moved into a solenoid coil with a restoring force of a spring used to move the cylindrical element out of the coil when de-energized. By adding permanent magnets to the structure, a linear force motor may be configured that allows the armature to be electrically driven in both directions with low input power.
Current industry designs for the force motors used in servo valve systems use a permanent magnet and a set of coils to superimpose magnetic fields at two air gaps to create a differential magnetic force. The coils produce a magnetic field directed through a cylindrical volume and permanent magnets produce a magnetic field that interacts with the magnetic field of the coils. The permanent magnet is set outside the cylindrical armature that is stational with the coils configured as the force motor of the first stage. In prior art designs, the force motor is external to a pilot valve and coupled to a flap/nozzle valve structure for generating differential pressure in the two differential pressure fluid flow paths. The two differential pressure fluid flow paths are then coupled to the main servo valve where the differential pressure is used to move elements of the larger servo valve to amplify the hydraulic produced force for moving large objects. Having three independent separated stages limits the size of present servo valves. In many of these applications, the size of the present servo valve structures limits the applicability of hydraulic servo systems.
When servo valves using a pilot stage with an embedded force motor are used in high vibrational applications the armature needs to be stiffer to resist deflections that may lead to increased wear and reduce reliability of the sliding surfaces of the armature.
There is, therefore, a need for a servo valve structure with an embedded force motor that is more rugged while maintaining small size and low cost.